With regard to the reduction step of obtaining metallic titanium from titanium tetrachloride as an intermediate product among the steps for producing metallic titanium from titanium ore, so-called Kroll process is most commonly adopted industrially. The following will describe a process for reducing titanium in the Kroll process with reference to FIG. 22.
After titanium ore has been chlorinated to be processed into titanium tetrachloride which is liquid at room temperature beforehand, through a liquid titanium tetrachloride supply pipe 8, titanium tetrachloride is dropped to a tightly closed reduction reaction vessel 1, i.e., onto a reaction bath liquid 2 containing fused magnesium as a main component at an average temperature of about 800° C. which is stored beforehand at the bottom of the reaction vessel 1. Then, highly pure metallic titanium is obtained by chemical change of magnesium into magnesium dichloride and of titanium tetrachloride into metallic titanium through chemical reaction in the reaction vessel.
Metallic titanium precipitates as fine particles on the bottom of the reaction vessel and then the particles are sintered each other to form a porous sponge titanium mass 4. Moreover, magnesium dichloride as the by-product precipitates on the bottom of the vessel to form a magnesium dichloride bath 3 owing to its larger specific gravity than that of magnesium. By adequately discharging the magnesium dichloride bath to the outside of the vessel through a magnesium dichloride discharge pipe 9, the surface level of the reaction bath liquid is maintained within a certain range. After the accumulated amount of titanium tetrachloride dropped has reached a predetermined level, the reaction bath liquid and magnesium dichloride are discharged to the outside of the vessel and the sponge titanium 4 is taken out of the vessel as a product after separating the bath liquid remained in the voids by heating in vacuo. In the case of recent representative large scale reduction reaction apparatus, the size of the reaction vessel is up to a diameter of about 2 m, a height of about 5 m, and a distance from the outlet of the titanium tetrachloride supply pipe 8 to the reaction bath surface of about 1 m, and a little less than 10 tons of sponge titanium is produced at one batch production. As the titanium tetrachloride supply pipe 8 including the outlet, a pipe having a diameter of 20 mm or more is usually used in order to avoid blockage at the outlet of the supply pipe, and a pipe having an enlarged end may be used in some cases in order to extend the dropping range of titanium tetrachloride. On the other hand, since the supply of liquid titanium tetrachloride is 250 kg/m2·hr at most, liquid titanium tetrachloride does not fill the supply pipe completely and the flow runs down along part of the pipe wall and is dispersed as a gas-liquid two-phase flow. Therefore, even when an original pressure of a supplying system is about tens of thousands Pa, it is difficult for titanium tetrachloride after discharged to maintain the momentum corresponding to the original pressure of the supplying system owing to a large pressure loss at discharging. Liquid titanium tetrachloride after discharged from the titanium tetrachloride supply pipe 8 drops as a large number of nearly free-falling drops 7 of liquid titanium tetrachloride while dispersed into a range having a radius of several hundred mm on the reaction bath surface 6.
Since the formation of metallic titanium from titanium tetrachloride is accompanied by a strong heat generation, reduction of heat from the reaction vessel is an important problem in the production. The reduction of heat from the reaction vessel 1 is effected by cooling, e.g., jetting air toward the outer wall of the reaction vessel 1 or the like. However, it is known that a heat load is concentrated to the outer wall corresponding to the vicinity of the reaction bath liquid surface 6, i.e., a larger amount of cooling jet flow is necessary at the wall.
An industrial large reaction vessel is usually made of steel and the eutectic temperature of iron-titanium alloy is about 1080° C. Therefore, when an inner wall temperature of the reaction vessel exceeds the temperature, there arise problems that dissolution of the reaction vessel wall remarkably shortens the life of the reaction vessel and also dissolved iron contaminates product titanium. Accordingly, in order to maintain the wall temperature of the reaction vessel at the temperature or lower, the flow rate of titanium tetrachloride should be limited to a certain upper limit, which is the most serious bottleneck of productivity in the conventional operation. In the past, many efforts have been made for improving the productivity.
For example, JP-A-7-41880 discloses an attempt to accelerate the removal of heat by mist-cooling of the outer wall of the vessel. (The term “JP-A” as used herein means an “unexamined published Japanese patent application.) Moreover, acceleration of cooling is attempted by inserting a cooling pipe into the reaction vessel in Japanese Patent No. 2883905. Although these inventions achieve the effect of cooling to some extent, but expensive incidental facilities are required and the effect is extremely restricted, so that they cannot be drastic measures. In order to improve cooling efficiency fundamentally, it is necessary to carry out structural improvement of securing a wide heat transfer area. However, there arises a problem that it is necessary to make a heat transfer wall, which itself is a highly heat-resistive element, thicker in view of strength, but the heat transfer efficiency again decreases thereby.
In consideration of the fact that the heat load concentration to the wall of the reaction vessel near the bath surface induces difficulty in improvement of the productivity, JP-A-7-41881 aims at homogenization of the bath temperature by inserting a titanium pipe having numerical openings into the bath liquid and stirring the bath by discharging argon gas from the openings into the bath. In JP-A-7-41881, however, there is only a description that “the heat transfer and temperature distribution in the bath are improved and hence the dropping rate of titanium tetrachloride is increased to enhance the productivity”, which is induced by discharging of argon gas into the bath, and conditions such as the surface temperature of the bath liquid and discharging depth of argon gas in the bath are not clear at all, although the conditions influence the heat load concentration to the wall of the reaction vessel critically. Actually, based on the findings of precise investigation in actual operation by the present inventors, it was confirmed that a strong circulating flow existed in the reaction bath liquid in the actual operation. It was observed that the mere discharge of argon gas into the bath hardly generates a stirring enhancing effect but argon gas bubbles were filled with magnesium vapor in the bath and, after they were released into over-bath gas 5, the vapor reacted with titanium tetrachloride vapor in the over-bath gas, so that a phenomenon of inhibiting the productivity was observed owing to the increase of the temperature of the reaction bath surface 6. That is, the inert gas discharged into the bath may cause either the case of reducing the temperature difference in the reaction vessel or the case of enlarging the difference depending on its position in the bath to be discharged and the amount of the gas supplied, so that it is not always observed that the discharge of an inert gas into the bath may result in a bath temperature-homogenizing effect.
Moreover, for the purpose of stirring the bath liquid similarly, JP-A-7-252549 describes an attempt that a liquid titanium tetrachloride-supply pipe is inserted into the bath, titanium tetrachloride is discharged in the magnesium dichloride bath to form bubbles by vaporization of titanium tetrachloride, the vaporized titanium tetrachloride is chemically reacted with the reaction bath liquid mainly composed of magnesium which is present above the magnesium dichloride, and thereby the bath liquid is stirred by the ascending bubbles collaterally. However, JP-A-7-252549 does not aim at temperature homogenization of a high temperature region near the bath surface, which may form due to a large amount of chemical reaction-generated heat in the vicinity of the bath surface, by a stirring effect caused by a gas-bubbling effect. This is because JP-A-7-252549 describes as follows: “when TiCl4 is supplied to fused MgCl2 layer, bubbles of TiCl4 which start to react at the lowest layer of the fused Mg layer ascend in fused Mg until the reduction reaction is completed”, “the reduction reaction is started”, and “the heat of reaction generated therein is diffused into the upper layer of the fused Mg layer via fused Mg and also is transferred to the lower layer of the fused MgCl2 layer by precipitation of the formed titanium and descent of MgCl2. Therefore, localized generation of the heat of reaction does not occur”. Thus, JP-A-7-252549 clearly intends that the reaction of titanium tetrachloride be completed in the bath. Moreover, there exists the description of “furthermore, the fused MgCl2 layer and the fused Mg layer are stirred by the gas bubbling action of supplied TiCl4 and the temperature distribution thereof becomes more homogeneous” in JP-A-7-252549. However, in JP-A-7-252549, the bath liquid stirring effect by gas-bubbling in the bath is collateral and the bath liquid having a little temperature difference owing to almost homogeneous heat generation in the bath is absolutely postulated. Furthermore, JP-A-7-252549 lacks specific descriptions on gas-bubbling conditions and bath liquid stirring mechanism by gas-bubbling. In addition, JP-A-7-252549 originally postulates a bath liquid having a little temperature distribution and describes no homogenizing effect of the bath liquid temperature by gas-bubbling itself. The problem of the invention described in JP-A7-252549 at the application to actual operation is that a long and large apparatus is required in order to finish the reaction of titanium tetrachloride bubbles released to the bath liquid completely in the bath liquid during their ascent in the case of a large supplying flow rate of titanium tetrachloride which corresponds to the conventional operating condition.
As described above, all the methods for reducing the heat load concentration to the wall of the reaction vessel in the conventional apparatus cannot be industrially adopted in actual operations. The problem common to these improved technologies is that, although the concentration of heat load of the reaction vessel to the vicinity of the bath surface is widely known as an operational fact, an effective improvement cannot be found out because of insufficient investigation on the physical cause of such a heat load distribution. Therefore, in the invention, in order to solve the problem, the reaction field in actual operation is precisely investigated to elucidate problems of the conventional art as physical phenomena.
FIG. 24 shows results of the investigation on a fluid temperature distribution in the axial direction inside the reaction vessel. In conventional actual operation, a bath temperature reaches a maximum temperature of, e.g., about 1000° C. on the bath surface and decreases rapidly to an average bath temperature just below the bath surface. That is, a high temperature layer exists just under the bath surface. On the other hand, the over-bath gas shows a maximum temperature in the region near the bath surface of 300 mm above the bath from the bath surface within the reaction vessel including the bath liquid. The reason why the temperature of the over-bath gas can maintain a high temperature despite of continuous cooling from the surrounding reaction vessel is because reaction-generated heat in the over-bath gas is extraordinarily large. FIG. 23 shows a conceptual illustration of the reaction on the bath surface and in an over-bath gas layer. The titanium tetrachloride drops 7 dropped into the reaction vessel 1 vaporize when reach a drop-falling point 11 in the reaction bath surface 6 and flow in the over-bath gas. During the flowing, part of titanium tetrachloride comes into contact with magnesium 13 in the bath surface to be reduced, which is defined as “bath surface reaction”. Titanium tetrachloride which does not react is reduced by the reaction with magnesium vapor in the over-bath gas 12, which is defined as “reaction in the over-bath gas layer”. Since the reaction of the titanium tetrachloride vapor with magnesium in a gas phase is kinetically difficult to occur, the reduction reaction in the over-bath gas is considered to occur mainly on fine particles floating in the over-bath gas. Most of the reaction-generated heat generated on the bath surface is transferred to the bath liquid side. This is because surrounding molecular density by which the heat generated at the gas-liquid interface is transferred is overwhelmingly higher at the liquid phase than in the gas phase. On the other hand, the reaction-generated heat in the over-bath gas layer increases the gas temperature to a certain temperature and then is transferred to the bath surface 6 and the inner wall of the reaction vessel 1 as the form of radiant heat to maintain the surface temperature of the bath liquid and the inner wall temperature of the reaction vessel at a high temperature. Therefore, this becomes one cause of concentrating a heat load to the wall 18 of the reaction vessel near the bath surface.
FIG. 7 shows the ratio of reaction quantity of the bath surface reaction 13 and the reaction 12 in the over-bath gas layer. In FIG. 7, abscissa represents the surface temperature of the reaction bath liquid and ordinate represents the ratio of the reaction rate in the over-bath gas to the total reaction rate. In the conventional art, the surface temperature of the reaction bath liquid is a high temperature of about 1000° C. and hence most of titanium tetrachloride dropped reacts in the over-bath gas. This is because saturated vapor pressure of magnesium shown in FIG. 6 is as high as the pressure of the over-bath gas owing to the high surface temperature of the reaction bath liquid in the conventional art and a large quantity of magnesium vapor is generated from the vicinity of the bath surface into the over-bath gas layer to cover the bath surface. Most of titanium tetrachloride vapor reacts with the magnesium vapor above the bath surface. As a result, titanium tetrachloride is consumed in the reaction with the over-bath gas before reaching the bath surface, so that the bath surface reaction hardly occurs.
FIG. 25 shows a flow field of the reaction bath liquid having such a heat distribution. As described above, a high temperature region exists just under the bath surface but this fact does not mean stillness of the bath liquid. Actually, a thin circulating flow 14 just under the bath surface exists in the perpendicular direction of the bath surface just under the bath surface, which results in temperature homogenization of the bath in the range that the circulating flow exists. In the case of a large reduction reaction vessel having a diameter of about 1 to 3 m, the thickness 15 of the circulating flow 14 just under the bath surface is about 100 mm or less. The “thickness 15 of the circulating flow just under the bath surface” means the length of the reaction bath range, wherein a circulating flow exists, in the depth direction from the bath surface, the circulating flow being generated by stirring the reaction bath liquid under the bath surface by imparting a compulsive force or natural convection, being in touch with the bath surface, and being circulated in the perpendicular direction in a time-average manner. The thickness of the circulating flow just under the bath surface can be determined by measuring the distribution of local time-average rate of the bath flow under the bath surface or by computing numerical values. The bath flow rate can be measured by inserting a current meter such as a Taft-type current meter or a Karman vortex current meter. Moreover, the thickness of the circulating flow just under the bath surface is not so influenced by the bath depth within the range of the bath depth of 1 to 5 m in a large reduction reaction vessel. Therefore, the value of about 100 mm or less for the thickness of the circulating flow just under the bath surface can be regarded as a common value in the conventional art. The force driving the circulating flow 14 just under the bath surface is as follows: firstly natural convection based on the temperature difference between a low temperature part of the bath liquid formed by cooling in the vicinity of the reaction vessel and a competitive high temperature part of the bath liquid formed by the reaction-generated heat in non-peripheral part of the bath and the radiant heat from the over-bath gas; secondly an upward stirring force based on the ascent of isolated bubbles (derived from titanium tetrachloride vapor+over-bath gas included into the bath) 20 at the drop-falling part 11 shown in FIG. 27. Since the titanium tetrachloride drops 7 to be dropped has a low rate almost near to free-fall, they cannot penetrate deeply into the bath liquid and may be removed by reaction or vaporization from the region 11 just under the bath surface. As a result that the titanium tetrachloride drops can penetrate into the bath only to a depth of about 100 mm below the bath surface at most, the portion at which the isolated bubbles generate is limited to a thin position within 100 mm below the bath surface. In addition, other than the circulating flow 14 just under the bath surface, a large number of circulating flows exist in the bath but the circulating flows which are not incorporated with the circulating flow 14 just under the bath surface are called circulating flows 16 at a deep position in the bath. Among the circulating flows at a deep position in the bath, a flow having a very large upward rate of up to several tens cm/s (FIG. 25, 16 (a)) exists but it hardly influences the temperature distribution of the circulating flow just under the bath surface. The behavior of the circulating flow just under the bath surface is controlled mainly by an entering or discharging heat distribution near the bath surface and a dropping state of titanium tetrachloride.
The reason why the thickness 15 of the circulating flow just under the bath surface becomes thin will be explained with reference to FIG. 26. Isothermal lines near the inner wall of the reaction vessel 1 just under the bath surface are represented by (a), (b), (c), and (d) in the order from a high temperature line. As a result of cooling the bath liquid by the inner wall of the reaction vessel, the maximum temperature part just under the bath surface has a lower temperature than the bath liquid temperature in the inner region in the radial direction of the vessel (a), so that a force to cause a downward flow along the inner wall of the reaction vessel works on the bath liquid. Next, as a result that a high temperature region of the bath surface is attracted by the above flow and flows into the vicinity of the inner wall of the reaction vessel to mix with the surrounding bath liquid having a low temperature, the temperature near the inner wall of the reaction vessel has been not always lower than that of the inner region (b) and the downward driving force has not been generated at this point of time, but the downward flow still remains by inertia near the inner wall surface of the reaction vessel. Furthermore, in the region near the inner wall of the reaction vessel at a lower part of the reaction vessel, the temperature of the region finally becomes higher than that of the inner region by the high temperature bath liquid entering from the upside along the wall surface (c), and an upward driving force is reversely generated near the inner wall surface to block further descent of the circulating flow. At this position, the circulating flow cannot descend along the inner wall of the reaction vessel any longer and turns to the inside direction of the reaction vessel. Namely, the bath liquid just under the bath surface strongly resists the descend of the bath liquid because of its extremely higher temperature as compared with the other part, but a strong stirring force derived from cooling of the reaction vessel and dropping of titanium tetrachloride is always imparted, so that a thin and high-speed circulating flow is formed just under the bath surface. Actually, the stirring force based on the density difference (temperature difference) imparted to the bath liquid just under the bath surface is estimated to reach several hundred N. Therefore, in the case that it is intended to enhance stirring by imparting a stirring force into the circulating flow just under the bath surface, an expectable effect may be small unless an additional force of at least several hundred N similar to the driving force of the circulating flow is imparted.